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Micron- and Nanometer Scale Fluid Mechanics Research Program
Jeff Guasto & Kenny Breuer
Our research is focused around the general theme of near-wall optical measurements of micro- and nano-scale fluid phenomena with colloids and aerosols. Our lab uses several different optical diagnostic techniques including TIRFM (total internal refelction flourescence microscopy), LIF (Laser induced flourescence) and digital holography to investigate fluid, thermal and mass transport near fluid-solid interfaces. Projects include quantum dot thermal-fluid tracers in liquid and gas phase, characterization of electro-spray droplet emissions from a Taylor cone, general near-wall velocimetry issues and particle-surface adhesion dynamics of bio-molecule coated surfaces.
Quantum Dots for Velocimetry and Thermometry
 Quantum dots (QDs) are semiconductor nanocrystals (2-20nm diameter), which, unlike typical fluorescent dyes, are resistant to photo-bleaching and have emission wavelengths tunable with the particle diameter. QDs also exhibit a predictable intensity change in fluorescence emission with temperature. Small tracer particles, like QDs, are desirable in micro- and nano-fluidics to probe fluid motion and properties close to solid boundaries. However, the small particle size results in large diffusivity or Brownian motion, which makes particle identification difficult. Additionally, QDs exhibit fluorescence intermittency or “blinking” that further complicates particle tracking.
We use an imaging technique called total internal reflection fluorescence microscopy (TIRFM), which allows for high signal-to-noise-ratio visualization of QDs within less than 200 nm of a fluid-solid interface. We have also developed a tracking technique to compensate for large diffusivities, blinking and high particle concentration called statistical particle tracking velocimetry (SPTV). By measuring tracer particle displacements and intensities, we can simultaneously measure the fluid velocity, temperature and viscosity within 200 nm of the micro-channel wall. QDs have also been applied to aerosol imaging and tracking. Micron-sized aerosol droplets are generated by a piezoelectric crystal nebulizer containing a liquid solution of QDs (organic or inorganic). The resulting aerosols are then imaged and studied using fluorescence microscopy.
Particle Dynamics Near Surfaces
Total internal reflection velocimetry TIRV (using TIRFM) has been widely used to measure fluid velocities in the very near-wall regions of micro-channels for several applications including slip flows and electrokinetic flows. Our lab employs a 3D version of this technique for an array of applications including measurement of hindered diffusion and quantification of near-wall velocity measurement error in PIV and PTV.
We also use TIRFM to measure the time resolved 3D motion of bio-molecule coated particles and surfaces in a near-wall, shear flow, which exhibit complex adhesion, surface rolling and release dynamics. Leukocytes (white blood cells) dynamically adhere to blood vessel walls through the reversible reaction between PSGL-1 and P-Selectin on the leukocytes and blood vessels, respectively. Cell adhesion and rolling are important for normal immune surveillance and to initiate immune system responses. The system is modeled by particles conjugated with P-Selectin and a glass micro-channel substrate conjugated with PSGL-1. From the particle trajectories, it is possible to estimate chemical reaction rates of bond formation and bond forces.
 Holographic Imaging of Electro-sprays
Taylor cones are generated when a conducting fluid is placed in an electric field and the free surface of the liquid forms a conical shape. An instability of the cone apex gives rise to a high-speed (10 m/s) jet which eventually breaks up into micron-sized droplets. We are using digital in-line holography with high magnification for 3D imaging of these droplets. 3D intensity fields are reconstructed numerically from a single 2D holographic image to characterize the droplets and break-up patterns.
Our work is supported by AFOSR/PSI, Sandia National Laboratories and NSF.
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